Frequency responsive control system



J1. D. REID 2,402,083

FREQUENCY RESPONSIVE CONTROL SYSTEM June 11, 1946.

Filed sept. 3o, 1941 f 1:125 wm ,425,57 C121 123 i Awa 46 CavloL 125 122 Znvenfor V/ k QW I0/w12 M *Q 2?/ 65 Patented June 11, 1946 FREQUENCY RESPONSIVE CONTROL SYSTEM John nneia, Mount Healthy, ohio, assignor to Radio Corporation of America, a corporation of Delaware Application September 30. 1941, Serial No. 413,058

This invention relates to frequency-responsive control systems, and has for its object to provide an improved frequency-responsive control system whereby a device or circuit may be controlled in some predetermined manner, actuated or prevented from functioning, in response to one or more frequency components of a received radio signal Without being responsive to said components when received with other signals or components of the same signal.

It is also an object of this invention to provide an improved frequency-responsive control system in connection with a radio signal receiving system for effecting differing control functions selectively in responseto signals at frequencies contained within a predetermined band of frequencies to which the signal receiving system is responsive.

It is a further object of this invention to provide a radio signa] receiving system having a, con-Y trol functionv responsive to predetermined audio frequency signals in relatively narrow bands while preventing the occurrence of the control function in the presence of wider range audio frequency signals inc ludingsaid bands. In accordance with the invention, audio frequency signals in the frequency bands within which the energy of speech or music is maximum may be adapted to control a condition of operation of the-receiving system, such as the signal output to a loudspeaker, or to control associated apparatus, such as a facsimile printer or a, facsimile receiving system, without interfering with a normal function such as the reception and reproduction of broadcast programs, including music and speech, covering the full audio frequency band within which the control frequency or frequency bands are included.

It is also an object of this invention to provide a control system for operating a relay device in response to intermittent signals at a predetermined audio frequency or in a predetermined band of audio frequencies contained within the band of audio frequency signals inherent in music, and for preventing operation of said device in response to a wider band of audio frequencies including said intermittent signal frequencies.

Furthenin accordance with the invention, a frequency-responsive control system is provided in connection with the audio frequency channel of a radio signal receiving system, for deriving sol an audio frequency signal or band of signals for blocking the control system, and another audio frequency signal or band of signals for operating the control system. Either or both signals may 8 Claims. (Cl. P18-5.6)

be rectified to provide suitable control potentialsl which, when applied to the control system, prevent the latter from operating or responding when a blocking signal is being received, while the system is operative in response tc a control signal in the absence of the blocking signal. The operating signal may be an intermittent signal such as a facsimile framing signal or it may be the band of signal frequencies in which the maximum energy of speech is provided.

The invention will be further understood from the following description, when considered in connection withfthe accompanying drawing, and its scope is pointed out in the 'appended claims.

In the drawing,

Figure 1 is a schematic circuit diagram of a frequency-responsive control system embodying the invention as applied to a radio signal receiving system and recording apparatus,

Figure 2 is a graph showing curves illustrative of operating characteristics of the circuit of .Figure 1,

Figure 3 is a further schematic circuit diagram of a modification of the control system of Figure 1, and

Figure 4v is a schematic circuit diagram of a frequency-responsive control system responsive to speech and embodying the invention in its application to a signal receiving system.

Referring to Figure 1, any suitable signal receiving system or signal source represented by the rectangle 5, for producing audio frequency signais, is connected to an audio frequency amplifying channel including a power amplifier 6 from 1 which output signals are derived through an output circuit l. A selector switch 8 in the output circuit provides a connection with a terminal 9 for energizing a suitable output device, such as a'loudspeakcr I0, and an alternative connection with a contact II for supplying the audio frequency output to any suitable utilization apparatus such as a recorder or facsimile printer l2 through a circuit indicated at I3.

The facsimile printer is supplied with operating current from supply means represented by the circuit I4, under control of a suitable relay device comprising contacts I5 and an operating winding or coil I6 which is placed under. control of the audio frequency signal currents in the circuit ;,3 through a frequency-responsive control system I8'.

The control system or channel I8 comprises `two tuned circuits, one of which is responsive to frequency band. such as a band of audio frequency signals inherent to music.

The second tuned circuit is responsive to a control or operating frequency also within the audio frequency range, which frequency is characteristic of a signal applied to a controlled apparatus, such'as the facsimile printer I2. In the present example, this is the facsimile framing signal which comprises intermittent pulses. It may, however, be any suitable operating signal providing operation or control of the relay device at the output end of the control channel. The blocking signal prevents theoperation of the relay device whether the control signal is present or not.

In the present example, the first tuned circuit, connected with the audio frequency output circuit I3, comprises a tuning inductance 20, a tuning capacitor 2| and a current-responsive element 22 connected in series. The last is the heater or fila.- ment of a rectifier tube 23 having an anode 24 connected with a positive potential supply lead 2l from the -power amplifier 3 or other suitable source, and having a cathode 26 connected through an output impedance or resistor 21 with anegative power supply lead 23 from the same source. The resistor 21 is provided with a shunt bypass capacitor 23.

The second tuned circuit, also connected with the audio frequency output circuit I3, comprises a tuning inductance 30, a series tuning capacitor 3| and a current-responsive device 32 which is the heater or filament of an amplifier tube 33. The last is provided with an anode electrode 34, connected with the positive supply lead 25, and a cathode 35, connected through the operating winding I3 of the relay and the resistor 21 by way of a lead 31 to the negative side of the supply circuit 23. The negative supply lead is also connected to ground or chassis as indicated at 33,.

The control grid 33 of the amplifier tube 33 is connected to ground or to the negative terminal 4l of the resistor 21 through a grid lead 4I.. The filament 32 may be provided with a shunt resistor 44 for limiting the current flow therethrough. Both control circuits 20--2I--22 and the circuit 30--3I-32 are connected back to thel power amplifier or audio frequency source through groimd or chassis connections indicated at 4l.

The operation is as follows: A radio signal is selected, amplified and detected in the system l in the usual manner and is further amplified in the power amplifier I, the audio frequency output of which may be applied through the switch 3-3 to the loudspeaker I3 for normal broadcast reception.

The audio frequency output may be applied to the output circuit I3 through the switch connection 3--I I, from which it is supplied to the apparatus I2 and to the two control circuits in parallel. In the present example, the first control circuit is tuned to the midfrequency of the peak energy content of music which may be assumed to be of the order of 660 cycles, while the second control circuit is tuned sharply to the operating signal for the control system which is chosen as the facsimile framing pulse frequency of 2000 cycles. v

When the control frequency is present, that is, when the facsimile signal is received, the circuit 33--3l'32 responds to'the 20Go-cycle signai, causing the series circuit to resonate at that v frequency and producing sun'icient current to heat the filament 32 ofthe control amplifier 33. Piate current then nows from the anode 34 to the cathode 35 and thence through the operating winding I8 of the relay device causing the contacts I5 to close, thereby energizing the facsimile printer I2 and permitting operation thereof in response to the received facsimile signal.

If music is present with or without the strong facsimile framing pulse, the circuit 20--2I--22 responds to the peak energy content thereof, causing the heater 22 `to be energized and permitting current to flow from the anode 24 to the cathode 2B of the rectifier 23 and thence through the output resistor 21 which applies a strong negative bias between the cathode 35 and the grid 33 thereby cutting off the tube 33 so that, whether the facsimile signalis present or not, the relay device may not operate. Likewise, the control channel is blocked against operation in response to any 200G-cycle component present in the music received through the system.

With thisarrangement; the receiving system may be energized and maintained in operation for facsimilev reception without energizing the facsimile apparatus except when a facsimile signal is actually received. Thus the normal transmis4 sion of broadcast signals including signals in the range of the control frequency does not operate the system.

The operation of the system may further be understood with reference to Figure 2 showing a curve 46 representing the response of the circuit 20-2I-22 to the peak energy content of music in the region of 660 cycles and a curve 41 representing the response of the circuit 30--3l-32 to the facsimile framing'impulses at 2000 cycles.

The latter circuit is preferably sharper in its frequency response than the circuit 23--21--22 as indicated by the relative width of the two curves.

It will be noted in connection with Figure 2 that the facsimile signal also includes a facsimile picture signal in a frequency range above the framing signal and a facsimile synchronizing signal which is a steady signal of the order of 480 cycles which falls below the response range of the blocking circuit 20-2I-22. The facsimile synchronizing signal does not block the control system when receiving facsimile signals since the blocking circuit is tuned above the facsimile synchronizi'ng signal.

The thermal inertia of the heater 32 is such that it will maintain the tube 33 conducting during the reception of the 200G-cycle framing pulses, thereby maintaining the relay. contacts closed for facsimile signal reception and recording. Also, during the reception of facsimile signals or when the tube 23 is in operation in response to normal broadcastl signals, the thermal inertia of the fllament22, as well as the thermal inertia of the filament 33F is such that the system is not affected by static pulses.

In the present example, the main energy frequency of the normal broadcast or music is utilized as the blocking signal, while the operating or control signal is the facsimile framing pulse. Both signals are passed through series resonant circuits which cause a variation in a direct current to operate a relay device controlling apparatus which in this case is the facsimile printer. The latter receives its signal from the same audioV frequency channel as the loudspeaker,

y In another embodiment of the invention shown in Figure 3, a complete facsimile receiving equipment may be energized under control of the facsimile framing pulse signal of 2000 cycles, while the standard broadcast or music frequencies, also containingv the 20w-cycle component. are preyvented from operating the facsimile system, as

in the circuit of Figure 1. The same blocking frequency of 660 cycles is utilized. However, rectification of the signals in the two branch circuits is provided to obtain biasing control of an amplifier which in turn controls the relay device.

Referring to Figure 3, the audio frequency output from ya. broadcast or other suitable receiver 50 is applied through an audio frequency output circuit 5I to either a loudspeaker 52 or a frequency-responsive control system having an input, circuit 53, through a selector switch 54 which is shown in a position to apply the signals vto the circuit 53.

The latter circuit includes a. blocking signal circuit 55 coupled through a tuned transformer 56 to the circuit 53 and an operating or control signal circuit 51 coupled to the circuit 53 through a tuned coupling transformer 58. The transformer 56 is tuned broadly to 660 cycles in the present example, while the transformer 58 is tuned sharply to 2000 cycles. A rectifier 60 coupled to the circuit 55 and a rectifier 6| coupled to the circuit 51 are connected in opposition to a common output resistor 62 ,across which is connected a suitable bypass capacitor 63.

In response to normal broadcast signals including musicl the circuit 55 is energized, causing the terminal 64 of the resistor 62 to become positive and the opposite terminal 65 to become negative. The latter terminal is connected through a lead- 66 to the control grid 61 of a control amplifier 6,78, in the output circuit 69 of which -is connected the relay operating winding I6.

Positive anode potential is also caused to flow through a bleeder resistor and a bias resistor 1| to ground 13 to apply a negative bias between the grid 61 and the cathode 13 which is connected at a point between the two resistors 1I) and 1I. It will be noted that the terminal 64 of the resistor 62 is also connected to ground'as indicated at 14v` Thus, a xed negative biasing potential from the resistor 1I is applied to the grid 61 through a circuit which may be traced from the grid through the lead 66 to the terminal 65, thence through the resistor 62 to the terminal 64 and ground 14--12. In the absence of received signals, this biasing potential is such that the anode current is substantially cut oft or maintained below a value sunlcient to actuate the relay. In the presence of signals from the broadcast receiver in the broadcast range, or containing music, the biasing potential on the grid 61 in increased by the amount of the voltage which appears across the resistor 62 as a result of rectication, mainly of the 660- cycle band, applied'to the rectifier 60 from the transformer 56.

If. during the reception of broadcast signals, the 2000-cycle facsimile signal is received, it passes through the circuit 51 by way of the sharply tuned transformer 51 and is rectified in the rectifier 6I. The rectied signal component therefrom is in opposition to that provided from the rectifier 60 and may reduce the negative biasing potential on the grid 61. However, the fixed bias 1I is such that it maintains the tube 68 biased off or below an anode current value sufficient to operate the relay.

In the absence of broadcast signals and in the presence of a received facsimile signal or other controlling signal to which transformer 58 is tuned, the rectifier 6I delivers a positive potential on the grid 61 which may have a value sufficient to overcome the negative bias from the source 1I and to permit the anode current to increase through the circuit 69 to the relaying value for f the relay winding I6, thereby causing the contacts I5 to close, and energizing apparatus controlled thereby which may, in the present example, consist of a facsimile receiver 15 and a facsimile printer 16 energized through the contacts I5 from a power supply line 11. In the present example, it will be seen that two control circuits are employed in the control system, one responsive to a control frequency and the other responsive to a blocking signal. With this arrangement, asin the circuit of Figure 1, the controlled apparatus is prevented from operating in response to signals in the audio frequency range,

valthough thel control frequency is included in that range. Normally, a blocking frequency band is selected which provides maximum energy in the normally received signal range.

In the present example, the time constant of the circuit is established by the diode out-put resistor 62 and shunt capacity at 63, Whereas in the circuit of Figure 1 the capacity at 29 and the resistor 21, in conjunction with the thermal heating time of the filaments, determine the time constant of that control circuit, both being such in the present example that the 2000-cycle framing pulses permit the relay I6-I5 to operate without interruption, in the absence of the blocking signal.

The control system thus far described isparticularly applicable for'the control of facsimile receiving apparatus whereby whenever a radio transmitter broadcasts a facsimile signal on the wave length to which a receiver is tuned, the

facsimile apparatus may operate to record the' transmission without being put into operation by the normal broadcast transmission from the same station. Thus, the system may be placed- 'in operation to stand by for facsimile signals,

without response to normal broadcast transmis sion although the controlling frequency is included therein. vA control system embodying the invention may, however, be provided with more than two control circuits and may be utilized to control apparatus other than facsimile apparatus. For example, it may be used to control a radio receiving system whereby transmission of signals of a certain character may be altered or cut off. One application of this arrangement is shown in Figure 4, to which attention is nowdirected.

A signal receiving system indicated at lis provided with an audio frequency amplifier 8| and a loudspeaker 82 coupled thereto, which may be cut off or muted in the presence of speech transmission alone as distinguished from the full transmission of music or other signals in the audio frequency band, thereby permitting the receiving system to respond only to music. .or other broadcast signals having substantially the full audio frequency range while being nonresponsive to transmission of speech alone.

Referring to Figure 2 along with Figure 4, the peak energy content of speech of occurs normally between 300 and 600 cycles, as indicated at 83. A control frequency band 84 below the normal speech band anda control frequency band 85 above the speech range may be'chosen to lprovide two blocking signals for preventing operation of the control system when musical sounds are received in either,` or both control bands. Control bands of from 30 to 90 cycles, as at 84, and from 2000 to 6000 cycles, as at 85, are suit; able for maintaining the control system blocked and the sound output from the receiving system 1s maintained although signals in the speech range may be received therewith. Thus, music, or other audio frequency output including the speech range, is prevented from operating the control system except when the output from the receiving system is wholly speech, or signals of other predetermined frequency characteristic containing no components of appreciable magnitude in either of the blocking frequency bands.

Referring again to Figure 4, a three-circuit control channel 90 is coupled to the audio frequency circuit 89 ofthe-receiving system through a suitable amplifier indicated by the single stage 90, the output circuit 9| of which is coupled to three-tuned coupling transformers indicated at 92, 93 and 94. The tuned secondary windings 95 and 96 of the transformers 92 and 93 are connected in series, aiding relation to a rectifier circuit 91 comprising a diode rectifier 98 and an output impedance 99 therefor. Transformers 92 and 93 are tuned respectively to the lower and the upper control bands or blocking signals which in the present example include the 30 to 90-cycle band for the transformer 92 and the 2000-6000 cycle band for the transformer 93.

With the arrangement shown, audio frequency signals received with a normal amplitude value in either or both of said bands are impressed upon the circuit 91 and the rectifier 98, resulting in a rectified direct current component across the resistor or output impedance 99. The polarity of the potential derived therefrom is indicated at the terminals of the resistor. The positive terminal is connected to ground and through ground, and a sourceof biasing potential provided by a resistor |02, to the 'cathode |03 of an output amplifier |04 .for the control chan.- nel.

The negative terminal is connected through a lead |06 to one grid |01 of the amplifier, which is of the pentode type. The other elements of the amplifier include a screen grid |09, a second control grid |09 and an output anode |I0. In the absence of signals, a fixed negative biasing potential is applied tc the grid |01 from the source |02 which forms part of a bleeder connection |||||2 from a positive anode supply lead ||3 to which the anode ||0 is connected through the operating winding I6 of the relay.

For controlling the output of the receiving system and by way of example, the relay contacts I0 are connected across the loudspeaker output circuit ||4 through a circuit lead ||5 and ground lli, although the muting control may be effected in any other suitable manner.

The time constant of the rectifier circuit 91- 90-99 is established by the resistor 99 and a shunt capacitor ||9 whereby the circuit is readily responsive to musical frequencies in the ranges indicated, to provide additional negative biasing potential to that provided by the source |02, thereby to maintain the amplifier |04 substantial- 1y at cut-od and the relay I6 inoperative in the presence of music or signals covering the maior portion of the audio frequency band.

The second control grid |09 is connected with the third control circuit comprising the tuned transformer 94 which is broadly responsive to signals in the peak energy range for speech, as between 300 and 600 cycles. Speech energy between these frequency limits is applied to a rectier circuit ||9 comprising a diode rectifier |20,

and an output impedance or resistor |2| shunted by a suitable capacitor |22 for establishing a pre- 8 determined time constant for operation 0f the rectifier circuit.

The positive terminal |23 of the rectifier output impedance |2| is connected through a time constant delay network, comprising a series resistor |24 and `a shunt capacitor |25 to a grid leak |26 connected with the other control grid |09. The negative terminal |21 is connected to ground |28 and lthence to the cathode |03 through the fixed biasing potential source |02. Thus, in the presence of a strong frequency component in the major portion of the speech range, the biasing potential provided on the grid |09 from the source |02 is overcome or reduced by the potential across the impedance |2| as a result of rectifying the signals in that band through the circuit ||.9 and rectifier |20.

In the event that signals in the two blocking frequency ranges are absent, theA increase in positive biasing potential on the second control grid |09 is sufficient to permit anode current to flow through the relay winding I6, thereby closing the contacts I5 and cutting olf the sound output to the loudspeaker 82.

However, in the presence of music, the channel through the circuit ||9 and the coupling transformer 94 is ineffective to cut off the loudspeaker because of the presence of signals in the circuit 91 which provides a negative biasing potential on the first control grid sufficient to cut off the tube |04 and to prevent the anode current from flowing at a value sufiicient to actuate the relay.

The time constant of the control network 99- ||8 is such that the circuit 91 is relatively fast acting, whereas the time constant of the network |2||22 is longer so that the circuit ||9 is slower acting than the circuit 91. The network |24-| 25 may be faster acting with a shorter time constant than the circuit network 99| I9. This enables the control to remain closed during pauses common to speech and yet to open rapidly upon resumption of music.

In the circuit of Figure 4, two blocking frequencies are provided, either or both of which impart to the control system a biasing potential for cutting off the output from the control channel, thereby rendering it inoperative. The control signal may be an intermittent signal of predetermined audio frequency or a band of frequencies. In any case, however, while the control frequency lies within the audio frequency range, music and other signals in the full audio frequency range are prevented from operating the control system by the proper selection of the blocking frequencies which are also derived from the same audio frequency channel of the receiving system, preferably in the band of frequencies outside that representing the peak energy con tent of speech. The control frequency from the same audio frequency channel is a different frequency.y In case the control frequency is intermittent, the thermal inertia of the rectifier or amplifier tubes or proper choice of time constants in the control channel of the rectifier circuit may be utilized to provide steady operation.

It will be vappreciated that the control of the output. through the rectifier 99 and circuit 91 may be made responsive to any suitable bands of audio frequency signals outside of the pass band of the circuit ||9 and likewise the control provided by the rectifier`|20 and the circuit ||9 may be made responsive to the music or other frequency band in the audio frequency spectrum so that, for exampie, the control circuit may function when ever music is present in the output of the receiving system, but does not function for speech alone, thus muting the receiver for music, but not for speech.

A control system embodying the invention is particularly effective in connection with a broadcast signal receiving system to control a condition of operation thereof or the operation of yan associated apparatus or circuit in response to signals of a predetermined audio frequency contained within the audio frequency band of the system without being operated by normal signals representing the full musical range of the system.

I claim as my invention:

l. A frequency responsive control system comprising in combination, an audio frequency signal channel, a tuned branch circuit coupled to said signal channel responsive to a predetermined frequency characteristic in an audio frequency band, a second tuned branch circuit responsive to a control frequency within said audio frequency band, a relay device, means including an electron discharge path for causing said relay device to operate in response to signals through said second branch circuit,and means to impede conduction through the discharge path for preventing the operation of said relay devicein response to signals through said first branch circuit.

2. The combination with a radio receiving system having an audio frequency output circuit, of a frequency responsive control system connected with said circuit comprising a tuned branch circuit responsive to audio frequencies in a band or audio frequency l0 cutting on the audio frequency output to said device, an amplifier tube for controlling said last named means, a circuit connected with said signal channel and responsive to frequencies within the range of speech for controlling said amplilfier and the output of said sound producing device, a second circuit responsive to audio frequency signals outside the frequency range oi' speech, connected with said amplifier tube to prevent conduction therethrough in response to said last named signals, whereby said system is selectively responsive to music and nonresponsive to speech. 1

6. A frequency responsive selective switching system comprising in combination, an audio frequency signal conveying circuit, s, tuned circuit coupled thereto responsive to a control signal at a frequency contained within a band oi' musical sound frequencies, a second tuned circuit coupled to said audio frequency circuit responsive to audio frequency signals in a band outside the major por- 'tion of the musical sound frequency range, a signal rectifier connected with each of said coupled containing maximum energy for music, a second branch circuit tuned to the framing pulses of a facsimilelsign'al within the audio frequency band, an electron discharge tube responsive to the reception of the last named signals to cause operation of a control device, and means responsive to signals in said first named branch circuit for biasing the electron discharge tube to prevent the operation of said control device.

3. The combination with an audio frequency amplifier having an output circuit, of means for deriving therefrom audio frequency signals in a predetermined band, a frequency responsive system, an electron discharge tube, means for applying said derived signals to said system to block 4the conduction of said discharge tube, means for deriving audio frequency signals from said output circuit in a second band of'audio frequencies, and means for applying said last named signals to said system -for effecting conduction of said discharge tube in the absence of signals in said rstnamed circuit.

4. The combination with a radio signal re ceiving system having an audio frequency amplifying channel, of means connected with said channel for deriving therefrom signals of apredetermined audio frequency contained within the band of audio frequencies inherent to music, an electron discharge tube, a .controlled apparatus responsive to current through said tube, means for controlling the functioning of said electron discharge tube including a circuit connected with said audio frequency amplifying channel and responsive to said predetermined audio frequency within the said audio frequency band, and means responsive to signals in said band other than said predetermined frequency for biasing said tube for preventing the operation of said controlled apparatus.

5. The combination with a radio receiver hav. ing an audio frequency signal channel, of a sound producing device connected therewith. means for winch the first-named band Dm circuit is re circuits for individually producing control potentials in response to signals applied thereto, and

selecting switching means including a relay and a controlling amplifier tube for said relay, said tube having a pair of control electrodes individually responsive to the control potentials produced by said rectiflers.

7. The combination with a signal receiving system, of an audio frequency ampliner circuit connected therewith for conveying signals therefrom, control potential responsive means for muting said circuit including an amplifier tube, a rectier circuit coupled to said first named circuit and having an output bias potential control connection with said amplifier tube, said rectifier circuit being responsive to audio frequency signals in'said irst named circuit in a predetermined frequency band, means responsive to audio frequency signals outside said band for blocking the controlfunction of said rectifier in the presence of musical sound frequencies, and a relay device connected in circuit with and controlled by said tube.

8. A selective switching system for facsimile receivers and the like, responsive to audio frequency impulses of a predetermined frequency contained within the audio frequency spectrum, comprising an amplier tube, a relay device operative in response to the anode current variations., of said tube, means providing a fixed biasing potential for said tube to prevent operation of said relay device in the absence of signals, a band pass audio frequency circuit. means for applying audiol frequency signals thereto, a rectiiler'connected with said band pass circuit having an output circuit connected with said bias `supply means to effect variation in the bias poe tential applied to said. amplifier in the presence of signals within the pass band of said last named circuit, a second band passaudio frequency circuit coupled to the signal means of said first named band pass circuit and having a rectifier connected therewith in polarity opposition with the nrst named rectifier. thereby to cause opsponsive.

JOHN D. REID. 

